In addition to performing therapeutic operations, conventional implanted cardiac stimulation devices may monitor and transmit cardiac electrical signals (e.g., intracardiac electrograins) to an external diagnostic device to observe electrical activity of a heart. It is common for implanted cardiac stimulation devices to send intracardiac electrogram signals to a monitoring device, such as an external programmer, to allow a user to analyze the interaction between the heart and the implanted device. Often the user can designate that the communication from the implantable device to the programmer include a transmission of codes which signal the occurrence of a cardiac event such as the delivery of a stimulation pulse or a spontaneous cardiac depolarization.
For example, U.S. Pat. No. 4,223,678, entitled "Arrhythmia Recorder for Use with an Implantable Defibrillator", issued to Langer et al. on Sep. 23, 1980, discloses an arrhythmia record/playback component within an implantable defibrillator. ECG data is converted from analog to digital form and stored in a first-in, first-out memory. When the defibrillator detects an arrhythmia event, it disables the memory so that no further ECG data is recorded in the memory until a command is received from an external monitoring device. This command requests the implantable defibrillator to transmit the stored ECG data to the monitoring device via telemetry.
Langer et al. in U.S. Pat. No. 4,407,288, entitled "Implantable Heart Stimulator and Stimulation Method", issued Oct. 4, 1983, discloses a programmable, microprocessor-based implantable defibrillator which senses and loads ECG data into a memory via a direct memory access (DMA) operation. A processor analyzes this ECG data in the memory to detect the occurrence of an arrhythmia event afflicting a patient's heart. Upon such an event, the defibrillator may generate a therapy to terminate the arrhythmia event and store the ECG data sequence, which corresponds to the arrhythmia event, for transmission to an external monitoring device and later study. In normal circumstances, when no arrhythmia event is occurring, the defibrillator continuously overwrites the ECG data in the memory.
U.S. Pat. No. 4,556,063, entitled "Telemetry System for a Medical Device", granted to D. L. Thompson et al. on Dec. 3, 1985, teaches a pulse interval telemetry system which is capable of transmitting analog data, such as sensed intracardiac electrogram signals, without converting analog data to a digital numeric value. The Thompson et al. telemetry system is capable of sequentially transmitting both digital and analog data, individually and serially, in either an analog or a digital format, to a remote receiver.
Causey et al., in U.S. Pat. No. 4,809,697, entitled "Interactive Programming and Diagnostic System for Use with Implantable Pacemaker", issued Mar. 7, 1989, disclose a programmer system for use with implantable programmable pacemakers that offers communications and diagnostic capabilities, including real-time and prospective analysis of the pacemaker operation, as it interacts with the heart. A graphical display is presented which illustrates the source of stimulating pulses in the atrium or the ventricle, the response or lack of response of the heart to such pulses and the existence and duration of refractory periods, for example. This capability is useful for understanding and analyzing the complex operation of dual chamber pacemakers. In particular, the improvement offered by the Causey et al. system over previous programmers is a display capability which includes a programmed time interval screen for illustrating a graphical or tabular representation of the programmed time intervals associated with the operation of an implanted pacemaker, allowing a user to quickly and easily understand and visualize the interaction between the timing of implantable device events and the body functions it monitors or controls.
Recently, cardiac pacemakers have included rate adaptive sensors to determine an appropriate rate for pacing a patient according to metabolic demands of the body. For example, U.S. Pat. No. 4,702,253, entitled "Metabolic-Demand Pacemaker and Method of Using the Same to Determine Minute Volume", granted to T. A. Nappholz et al. on Oct. 27, 1987, discloses a rate-responsive pacemaker which senses impedance in the pleural cavity of a patient and derives respiratory minute volume as a function of the measured impedance. In turn, the respiratory minute volume measurement is correlated with pacing rate, the greater the amount of air exchanged, the greater the need for a higher pacing rate. U.S. Pat. No. 4,901,725, entitled "Minute Volume Rate-Responsive Pacemaker", issued to T. A. Nappholz et al. on Feb. 20, 1990, teaches an improved minute volume rate-responsive pacemaker, which also sets pacing rate according to minute volume measured using an impedance measurement. U.S. Pat. No. 5,197,467, entitled "Multiple Parameter Rate Responsive Cardiac Stimulation Apparatus", issued to B. M. Seinhaus et al. on Mar. 30, 1993, discloses an apparatus which employs multiple physiological rate control parameters, which are analyzed to determine the best pacing rate. The frequency content of the measuring current determines the respective parameter being measured.
One problem with rate responsive pacemakers which employ a sensor to sense a physiological parameter is that, after long term implantation, the time waveform of the sensed signal is not available for visualization, measurement or analysis. There has been no way to monitor changes in the sensed signal over time which may occur as the lead/tissue interface matures. Furthermore, the inability to monitor the sensor signal has led to some confusion concerning what the true physiological basis is for the signal which is sensed. For example, some clinicians have suggested that the respiration measurement for a minute volume rate-responsive pacemaker actually measures patient motion or vibration, rather than respiration. E. Alt, in U.S. Pat. No. 5,003,976, entitled "Cardiac and Pulmonary Analysis via Intracardiac Measurements with a Single Sensor", granted Apr. 2, 1991, compounds this confusion by showing that a single sensor can sense different physiological signals, as well as signals having a non-physiological basis, depending on the application of different filters to the incoming sensed signal stream.
It is desirable, therefore, to have the capability in an implantable cardiac stimulation apparatus to transmit sensed physiological signals to an external monitoring device for display and analysis.
Accordingly, it is a primary object of the present invention to provide an improved apparatus and method for sensing physiological signals based upon a measurement of body impedance, formatting these signals and transmitting the formatted signals to an external monitoring device, such as a pacemaker programmer, for display.
It is a further object of the present invention to provide an improved capability for visualizing physiological signals to identify a physiological basis for the origin of such signals and confirm that impedance signals, thought to respond to respiration, the forces of cardiac contraction and body motion, do actually respond to these sources.
It is a still further object of the present invention to provide an improved capability for the visual monitoring of physiological signals over time to allow observation of changes in such signals in a chronically implanted device.
It is an additional object of the present invention to provide an improved capability to identify noise sources which affect a physiological signal to allow for the design of filters to remove or reduce the influence of noise signals.
It is another object of the present invention to provide, in a long term implanted rate adaptive heart stimulation device, an improved capability to externally monitor physiological signals over time to calibrate the transfer function for mapping the physiological signal characteristics into a stimulation rate.
It is an accessory object of the invention to transmit a physiological signal to an external monitoring apparatus to allow diagnostic testing of a physiological sensor, leads and measuring circuit.
It is a supplementary object of the invention to provide raw data for physiological research, such as research relating to respiratory ambulatory physiology.
It is an important object of the present invention to provide for simultaneous transmission of combined intracardiac electrogram and physiological signals.
It is an object of the present invention to provide for simultaneous transmission of combined intracardiac electrogram and physiological signals to supply information, additional to cardiac electrical signals, regarding placement of leads during implantation.
It is a supplemental object of the present invention to provide for simultaneous display of intracardiac electrogram and respiration signals to supply information allowing the detection of arrhythmia conditions. For example, an increase in minute volume shown in the respiration signal indicates an exercise condition of a patient. An increase in heart rate concurrent with an increase in minute volume measurement indicates that the fast heart rhythm results from exercise rather than a pathological arrhythmia.
It is a still further object of the present invention to provide for a simultaneous display of intracardiac electrograms and mechanical cardiac contraction waveforms, which indicate a measure of cardiac contractility, denoting the ability of the heart to pump in response to propagated, stimulating electrical waves.
Further objects and advantages of the invention will become apparent as the following description proceeds.